Record goal-aligned Optimiser objectives as ADR-0062

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/adr/0062-goal-aligned-optimiser-objectives.md

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+---
+status: accepted (extends ADR-0016; composes with ADR-0061)
+---
+
+# Goal-aligned Optimiser objectives: each goal maximises its own metric
+
+Every Scenario goal used to optimise SAP. The legacy engine returned no
+target for Energy Savings / Reducing CO2 (`optimiser_functions.calculate_gain`
+→ `None`) and maximised SAP gain within budget regardless of the goal, and the
+new engine inherited that: the goal label changed nothing but the words on the
+brief. The scorer already computes each package's carbon and (via SapResult →
+EnergyBreakdown → BillDerivation) its annual bill, so aligning the objective
+is a selection change, not a calculator change.
+
+Decided in a grilling session with Khalim, 2026-07-09.
+
+## Decision
+
+**The Optimiser maximises the Scenario goal's own metric, as a pluggable
+`objective: Callable[[Score], float]` (higher is better), with no target:
+goal-aligned briefs are "reduce as much as possible within this budget".**
+
+- **Reducing CO2 emissions** maximises annual kg CO2 saved
+  (`-score.co2_kg_per_yr`).
+- **Energy Savings** maximises the annual bill £ saved, priced at the **live
+  Fuel Rates snapshot** (ADR-0014), not SAP's internal tariff book — that
+  difference is the point of the goal. SAP is itself a cost-shaped rating, so
+  the two frequently agree; they diverge exactly when current tariffs disagree
+  with SAP's assumptions (e.g. the gas/electricity price ratio).
+- **Increasing EPC** keeps its SAP objective and band-target semantics
+  (least-cost-to-target, repair, max-gain fallback) unchanged.
+- **Valuation Improvement / None** stay max-SAP-within-budget — SAP is a
+  defensible valuation proxy and `None` has no semantics to encode.
+- **`goal_value` is ignored for the goal-aligned goals** — no percentage or
+  absolute target exists yet. If targets arrive later they slot into the
+  existing target machinery on the objective's scale.
+- **A budget is mandatory** for the goal-aligned goals: unconstrained
+  "as much as possible" would recommend every beneficial measure. A
+  budget-less Energy/CO2 Scenario raises a `ValueError` naming the scenario
+  and goal — a loud misconfiguration, not a maximal plan.
+- **One currency everywhere**: the role-1 group signals
+  (`independent_option_signals`), the forced Measure Dependency pricing, the
+  greedy-repair marginals, and Fabric First's phase-2 re-scoring
+  (ADR-0061) are all measured by the same objective, so a ventilation that
+  costs SAP but is carbon-neutral cannot sink a carbon-improving wall, and a
+  fabric-first phase 2 picks its heating on post-fabric carbon, not
+  post-fabric SAP.
+
+## Consequences
+
+- Selection changes; truth-telling does not. The Plan's persisted Scores,
+  Bills, and role-3 SAP attribution are computed exactly as before — only
+  *which* package is chosen responds to the goal.
+- At a £16,000 budget on the uninsulated solid-brick corpus dwelling
+  (001431), the SAP objective buys wall + floor + gas boiler (SAP 72.9,
+  2,069 kg CO2/yr, £2,088/yr) while the carbon objective buys wall + floor +
+  storage heaters (SAP 69.2, 1,098 kg CO2/yr, £2,635/yr) — goals now trade
+  SAP, carbon and bills against each other visibly.
+- The Energy Savings objective inherits the Fuel Rates snapshot's staleness
+  characteristics (quarterly Ofgem-cap cadence, ADR-0014).
+- `independent_option_impacts` (role-1 SAP/CO2/kWh triple) is removed —
+  superseded by `independent_option_signals` in the objective's currency.